Gas tracer composition and method

ABSTRACT

The invention provides a method for tagging methane by adding thereto one or more of the tracer compounds sulfur hexafluoride and chloropentafluoroethane. The methane being tagged is noramlly being stored in underground storage fields to provide identity and proof of ownership of the gas. The two tracer compounds are readily detectable at very low concentrations by electron capture gas chromatography apparatus which can be made portable and thus suitable for use in the field.

BACKGROUND OF THE INVENTION

This invention relates to a gas tracer composition and method. Morespecifically, this invention relates to methods for the tagging ofmethane and to tagged methane compositions. The term "methane" is usedherein to denote not only pure methane, but also to natural gascompositions which are mainly methane but may contain minor proportionsof other gases, for example helium, ethane and hydrogen sulfide. Theterm "methane" as used herein also extends to so-called syntheticnatural gas i.e. methane produced by chemical synthesis and intended tobe used as a replacement for natural gas. "Methane" may also refer toliquified natural gas (LNG) as it is produced either domestically oroverseas. LNG is made by the pressurized and cooling liquifactionprocess as it is performed on natural gas coming out of the ground.

The demand for natural gas is of course largely seasonal, because muchof the natural gas is consumed for heating purposes. Since it isdesirable to maintain a steady output from gas wells throughout theyear, it is necessary to store enormous quantities of natural gas duringthe warmer months of the year in order to meet the peak winter demands,thereby to allow proper matching of the relatively steady production ofnatural gas against the fluctuating demand therefor. Because of theenormous quantities of gas (of the orders of billions of standard cubicfeet) which have to be stored, the construction of artificial storagefacilities for the gas is economically unattractive, and most of the gasis thus stored in natural formations, primarily exhausted natural gasproduction fields, though salt domes may also be used.

The geology of the exhausted gas fields used for storage may be bothcomplex and not entirely known; for example, unknown to the operatorsthere might be underground communication between an exhausted gas fieldused for storage and a nearby field which is still being used forproduction. Furthermore, there is a possibility that someone elsedrilling in the vicinity may penetrate the gas-containing formationsduring drilling operations, and proceed to produce gas from the storagefield, thereby (knowingly or unknowingly) stealing gas from its rightfulowner. Accordingly, it is very desirable that the owner of the gas beingstored in a gas storage field be able to "tag" the gas by adding a minorproportion of a tracer compound thereto, so that should the gas placedin the storage field migrate to a nearby field or be removed byunauthorized persons it can be identified; it will be appreciated thatgenerally natural gas is a fungible product and, absent the addition ofsome tracer compound thereto, it is not possible to determine the sourceof any particular sample of natural gas.

Hitherto, the main tracer compound used to tag methane has beenethylene. Until recently, ethylene was thought to be a good tracercompound because it moves through a gas field in a manner very similarto methane (and thus does not become separated from the methane as themethane diffuses through large gas fields) and because it was thoughtnot to occur in natural methane supplies. Furthermore, even very smallamounts of ethylene are easily detectable in methane. Unfortunately, ithas now been discovered that although ethylene does not occur naturallyas a contaminant of methane, there are at least two ways in whichmethane may be contaminated with traces of ethylene even though it hasnot been deliberately tagged with ethylene. Firstly, some fields nowused for methane storage have previously been used for storing coke-ovengas, which contains considerable amounts of ethylene. The storage ofcoke-oven gas in these fields has left a residue of ethylene within thefields, so that methane supplies pumped into such fields do becomecontaminated with this residual ethylene. Secondly, some of theprocesses by which natural gas fields are prepared for use as eitherstorage or production facilities produce ethylene; in particular,treatment of gas wells with acid to clean and open casing perforationresults in contact of the acid with the steel in the well pipe, thusproducing trace quantities of ethylene. Accordingly, ethylene is nolonger useful as a reliable tracer compound, and there is thus a needfor other tracer compounds for tagging methane.

The selection of appropriate tracer compounds for use in tagging methanesupplies presents very considerable difficulties, in view of the veryexacting requirements within a tracer compound must meet. Firstly, thetracer compound must be one which does not occur even in minutequantities in natural supplies of methane, which may contain a varietyof contaminants including the inert gases, hydrogen sulfide, ethane,propane and other hydrocarbons. In addition, the tracer compound mustdiffuse through a gas storage field in a manner very similar to that ofmethane, otherwise if a single injector well or a small cluster ofinjector wells are used to inject gas into a large storage field, thetracer may remain in the vicinity of the injector well(s), leaving gasat large distances from the injector wells effectively untagged. Sincemany gas storage fields contain traces of liquid hydrocarbons, thetracer compound must not be too soluble in such hydrocarbons, otherwisethe tracer will be absorbed by the liquid hydrocarbons, again leavingthe gas stored in the field untagged. The tracer compound must be stablefor periods of at least several months in the presence of methane andany possible impurities in natural methane supplies. Moreover, in viewof the enormous volumes of gas which have to be tagged (a single naturalgas field may typically store ten to fifty billion standard cubic feet(Bscf) of methane,) and the difficulties which may be encountered intransporting large quantities of tracer compounds to the gas storagefields, economic considerations dictate that any tracer compound bedetectable in concentrations of no more than a few parts per million inmethane. Since investigations of unauthorized removal of tagged naturalgas from a storage field may require analysis of the suspect gas in thefield, the tracer compound must be detectable at such very lowconcentrations in methane using readily portable apparatus. Finally,since it is not practicable to remove the tracer compound from themethane after it has been withdrawn from storage, the tracer compoundmust not interfere with or create dangers during any of the normal usesof methane.

It should be noted that the provision of tracer compounds for methanestored in gas storage fields involves very different problems from theprovision of tracer compounds which are used to detect leaks in gaspipelines. When it is desired to use a tracer compound to find theposition of a leak in a gas pipeline, only a relatively small amount ofgas flowing through the pipeline has to be tagged, and thus it ispractical to use much higher concentrations of tracer compounds. Indeed,since the leaking pipeline is frequently shut down, it may be possibleto fill the pipeline with the "tracer" compound in order to produce themaximum possible concentration of tracer compound adjacent the leak.Thus, many techniques for detecting leaks in pipelines are totallyinapplicable to tagging gas stored in storage fields. Furthermore,tracer compounds used for detecting leaks in pipelines do not have tomeet the requirements of (1) having a mobility through rock, (2) notdissolving in hydrocarbons and (3) being stable for months at a time,which a tracer compound intended for use in gas fields must meet. Forthis reason, most of the tracer compounds suggested for use in detectingleaks from pipelines are totally unsuitable for use in tagging methaneto be stored in gas storage fields. For example, U.S. Pat. No.3,523,771, granted Aug. 11, 1972, to Anderson, describes a process fordetecting leaks from pipelines in which there is added to a fuel gaspassing through the pipeline from 0.01 to 10% by weight of certainorganometallic compounds which react spontaneously with air to produce asmoke visible at the source of a leak in a pipeline. The use of theseorganometallic compounds as tracers in gas fields would be entirelyinappropriate since the amount of tracer which would have to be addedwould be so large as to be economically impracticable, and in any case,gas recovered from a natural gas storage fields almost never comes intocontact with air. Furthermore, it is most unlikely that suchorganometallic compounds would diffuse through a gas storage field atthe same speed as methane, and, as described in U.S. Pat. No. 3,523,771,the addition of organometallic compounds to the field gases does presentserious problems in later using the fuel gases unless thatorganometallic compounds are removed; while it is practicable to removethe organometallic compounds from the relatively limited amount of gaswhich has to be tagged for leak detection purposes, it is utterlyimpractical to attempt to remove a trace of compound from the hugequantities of gas recovered from a gas storage field. Accordingly,although the process described in U.S. Pat. No. 3,523,771 may be auseful method of tagging fuel gases to detect leaks in pipelines, it isentirely impractical as a method of tagging gas to be stored in a gasstorage field.

After extensive evaluation involving over 140 potential tracercompounds, we have now discovered two new tracer compounds for methanewhich meet all the aforementioned requirements.

SUMMARY OF THE INVENTION

This invention is based upon the discovery that sulfur hexafluoride andchloropentafluoroethane meet the requirements set fort above for tracercompounds for use in tagging methane. Both compounds are readilydetectable at concentrations of a few parts per million or below byelectron capture gas chromatography.

Accordingly, in one aspect this invention provides a method for taggingmethane comprising adding to the methane a minor proportion of sulfurhexafluoride or chloropentafluoroethane (Freon-115).

A further aspect of the invention is a tagged methane compositioncomprising methane and a minor portion of sulfur hexafluoide orchloropentafluoroethane.

Finally, the invention provides a method for tagging methane in anunderground storage field which comprises adding to the methane to bestored in the field a minor proportion of the tracers and pumping thetagged methane into the underground storage field via an injection well.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of the apparatus used for adding tracer compoundsto methane during the testing period.

FIG. 2 is a schematic of the apparatus for use in a liquid injectionsystem according to the present invention.

FIG. 3 is a schematic of the apparatus for gaseous injection systemaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the methods of the instant invention are primarily intended forthe tagging of gaseous methane in underground storage fields, theinvention is also applicable to tagging methane in other forms. Forexample, it may be used to tag methane stored in above-groundpressurized storage tanks, and liquified methane in road or railtankers, liquified natural gas storage tanks or liquified natural gastransport ships.

The two tracer compounds used in the instant method, sulfur hexafluorideand chloropentafluoroethane and may be used singly or in combination.Indeed, it is one of the advantages of the instant invention that, byproviding a plurality of tracer compounds and combinations of tracercompounds, it enables different masses of gas to be labeled differently.For example, in certain circumstances where two gas storage fields lieadjacent one another, it may be desired to label the gas in the twofields differently, and the instant invention renders it possible to doso.

The utility of sulfur hexafluoride and chloropentafluoroethane as tracercompounds is due to their easy detection at very low concentrations byvarious gas chromatography techniques. One method used was flameionization chromatography which is effective for detecting ethylene.Thermal conductivity chromatography can also be used in detection ofchloropentafluoroethane and sulfur hexafluoride, however detection ofsulfur hexafluoride by thermal conductivity chromatography is moredifficult when the concentrations are very low. Accordingly, thepreferred method of analyzing for the presence of sulfur hexafluoride iselectron capture gas chromatography. Electron capture gaschromatographic techniques are especially effective in the detection ofgood electron receiving molecules such as chloropentafluoroethane andsulfur hexafluoride, however ethylene is not capable of detection byelectron capture gas chromatographic procedures. Electron capture gaschromatographs are available commercially which are sufficiently compactand lightweight to be easily transported in a vehicle and which can givegood results under field conditions. Using such electron capture gaschromatographs, we have found that sulfur hexafluoride is detectable ata minimum concentration of about 5 parts per trillion andchloropentafluoroethane at a minimum concentration of about 0.35 partsper million. Because sulfur hexafluoride can be detected at lowconcentrations, and thus much less tracer compound is required to tag agiven quantity of gas, it is the preferred tracer for use in the instantmethod. Although the tracer compound should of course be added to themethane at a concentration considerably greater than the minimumconcentration, to allow for some dilution of the tagged methane by othermethane which may be present in the storage and to allow for morereliable detection, the amount of sulfur hexafluoride needed for properoperation of the instant method is very small; we prefer to use sulfurhexafluoride as a tracer compound in an amount not exceeding about fiveparts per billion of the methane by volume, and the optimumconcentration of sulfur hexafluoride appears to be about 500 parts pertrillion by volume. In contrast, we prefer to usechloropentafluoroethane in an amount not exceeding about 20 parts permillion of methane by volume, and most desirably about four parts permillion by volume.

The exact method by which the tracer compound is mixed with the methaneis not crucial. For example, a gaseous stream of the tracer compoundcould be mixed directly with a gaseous stream of methane being pumpedinto an underground storage field. However, because injection wells forunderground gas storage fields may be in relatively remote areas, andthus it may be necessary for apparatus adding the tracer compound tomethane being pumped into the storage fields to run unattended forseveral days, to reduce the capital cost and physical volume of theapparatus for storing and injecting the tracer compound, we have foundit convenient to store both tracer compounds in liquid form and to addthem to the methane as a mist of liquid tracer compound which of courserapidly evaporates to form a homogenous mixture of tracer compound andmethane. The provision of suitable liquid pumps for pumping appropriatequantities of chloropentafluoroethane does not present any problems,since the quantities of this tracer which need to be added to a typicalnatural gas stream entering a storage field are sufficiently large to bewell within the pumping range of commercially-available pumps suitablefor field use. However, the preferred concentrations of sulfurhexafluoride tracer are so low that a typical methane flow into anunderground gas storage field only requires the addition of about 5 ml.per day of sulfur hexafluoride as tracer compound, and it is difficultto find pumps suitable for field use which can pump liquid sulfurhexafluoride at this extremely low rate. Moreover, at such low flowrates even very small leaks may cause serious problems and, if even amodest length of tubing is positioned between the sulfur hexafluoridesource and the gas pipeline, the tracer will take a long time to reachthe pipeline and tag the gas therein. Accordingly, in many cases it willbe found more convenient to add sulfur hexafluoride tracer to methane inthe form of a liquid solution of sulfur hexafluoride in a liquidsolvent, such as methanol or carbon dioxide, in order to dilute thesulfur hexafluoride and allow for pumping of liquid at a rate which ismore suitable for commercially available field-usable pumps. Carbondioxide needs to be maintained at a pressure of 900 psi in order to becapable of liquifaction at reasonable room temperatures; however, carbondioxide has been found to be a very effective carrier and nostratification of sulfur hexafluoride in a liquid solution with carbondioxide has been found even following months of storage. FIG. 2 depictsschematically an equipment arrangement suitable for the injection ofsulfur hexafluoride in a liquid solution with carbon dioxide as a tracergas directly into a methane pipeline. The same equipment arrangement canbe utilized for the injection of chloropentafluoroethane which occurs asa liquid at room temperature without the necessity of it being resolvedin a carrier liquid.

FIG. 2 shows schematically an apparatus being used to carry out theinstant method. This apparatus includes a gas supply line 148 in apipeline 216 via which gas in provided from a pipeline (alternativelythe line 148 could be connected to a gas field).

Regulators 150, 152 and 154 progressively reduce and maintain the pumpoperating gas to a operable pressure of around 75 psig from the normalpipeline pressure of 600 to 1400 psi. Pressure gauges 156, 158, and 160reflect the various pressures that are being controlled by theirrespective regulators. A solenoid valve 162 is controlled from a maincontroller unit which allows for the unsupervised operation of thesystem; solenoid valve 162 is turned to an off position at times duringwhich the pump is not to be operated. An in-line filter 164 is providedsimply to clean the gas. From the filter 164, the gas passes through apump controller 166 and thence to a gas-powered liquid pump 168 of thepositive displacement type, which is controlled by the controller 166.After being used to drive the pump, excess gas is simply vented to theatmosphere. The tracer in a liquid form, either sulfur hexafluoridedissolved in liquid carbon dioxide or chloropentafluoroethane, is storedin bottles 170. The concentration of sulfur hexafluoride in the liquidsolution is predetermined based upon the size of the field beinginjected or the amount of gas to be tagged and is shipped to the fieldalready made up in appropriate concentration. When thechloropentafluoroethane is utilized it is used in a pure liquid formwithout a diluent liquid, the pump and its controller systems beingcapable of correctly metering the required amounts without the benefitof a diluting liquid. The liquid is withdrawn from bottles 170 throughisolation valves 172 by pump 168. A pressure gauge 176 is provided toindicate the pressure at the outlet from the bottles 170 and isespecially necessary when liquid carbon dioxide is utilized as thediluent liquid since a pressure of at least 900 psig must be maintainedin such a carbon dioxide diluted system. A further pressure gauge 178provides visual indication of the output pressure of the pump which mustbe high enough to allow injection into the field and also sufficientlyhigh to be properly regulated by a flow control valve 192 describedbelow. Downstream of the pressure gauge 178, a vent line 180 is providedfor purging the line of air during start-up of the pump 168. Next, anaccumulator 182 is provided to smooth flow along the line and reduce thevariations in flow caused by the discrete strokes of the positivedisplacement pump 168. A by-pass line 185 branches from the pump outletline at 184 to return unused tracer liquid to the bottles 170 forrecirculation. This by-pass line 185 incorporates a regulator 188 and anassociated pressure gauge 186 to ensure that tracer liquid isrecirculated via line 185 only when the amount of tracer liquid suppliedby the pump 168 is in excess of that required for tagging purposes. Fromthe branch point 184, the tracer liquid passes through a filter 190 andthence to the flow control valve 192; the filter 190 serves to preventobstruction of the flow control valve 192. The flow control valve 192 iscontrolled by a controller unit 210 so as to meter the appropriateamount of liquid; thus the flow of tracer liquid can be regulated from avery low flow or no flow all the way to metering in the proper taggingamounts for full flow in the pipeline 216. The controller unit 210additionally takes data from flow meters 196 and 214 having differentialpressure cells 198 and 212 respectively in order to regulate the amountof tracer liquid being passed by flow control valve 192 based uponactual flow measurements both in the main line and in the tracer line.The controller 210 is provided with a visual and audio alarm 258 toindicate when there is flow through the pipeline but no flow through thegas tracer line, thus indicating failure of the gas tagging system. Thepressure at flow control valve 192 is maintained such that there issufficient "subcooling" of the tracer liquid that no flashing of theliquid occurs in the control valve which is generally a high-pressuredrop device. Flashing of the liquid would render useless themeasurements by flow meter 196 and would not allow sufficient control ofthe tracer volumes in order to be assured of economical gas tagging. Apressure gauge 200 measures the pressure of the tracer liquid downstreamof the flow control valve 192. A regulator 204 steps down the pressureto an appropriate valve for injection directly into the gas pipeline216. A check valve 202 disposed between the pressure gauge 200 and theregulator 204 prevents backflow from the pipeline 216 back into thesystem especially during periods when no tracer is being injected. Avent line 206 vents any excess pressure found in the last leg of theline, while an isolation valve 208 isolates the entire liquid injectionsystem from the pipeline 216.

Additionally sulfur hexafluoride may be injected in a gaseous formutilize a diluent gas, preferably nitrogen. FIG. 3 depicts schematicallythe apparatus that may be utilized for gaseous injection of sulfurhexafluoride dissolved in gaseous nitrogen as a tracer gas into a gaspipeline 256. The mixed gas sulfur hexafluoride and nitrogen is storedin bottles 220 which are shipped to the field in a predeterminedconcentration based upon the amount of gas to be tagged or the size ofthe reservoir to be tagged. Isolation valves 222 are used to prevent theescape of gas from the bottles until it is required by the system. Thepressure of the bottles may be observed on a pressure gauge 224 which isassociated with the inlet side of a gas regulator 226. A vent 228 isprovided to prevent inadvertent overpressurization and also to vent theline after an injection has been completed. The pressure in the gas lineafter reduction of pressure by a regulator 226 is indicated by apressure indicator 230. An in-line filter 232 is provided to protect aflow control valve 234 from foreign matter. Gas flow control valve 234is controlled by a controller 250 which receives gas flow informationfrom both the pipeline and from the gas being injected through thetagging system. Flow meters 236 and 254 and their associateddifferential pressure gauges 238 and 252 provide such flow informationdirectly to the controller 250. The controller 250 is also provided witha visual and audio no-flow alarm 258 to indicate conditions when thereis flow through the main line but no flow through the gas tagging line.This is necessary so that an operator making his rounds in the field maybe aware of the fact that there is no tagging gas being injected intothe main gas stream even though such tagging gas may be required. It isnecessary that such an alarm be a lock-in alarm if this is an unattendedinjection station which it is anticipated will be the overwhelming useof such injection equipment since such injection points are usually atremote places. Downstream in the injection line from the flow meter 236is pressure gauge 240 which indicates the downstream pressure after theflow control valve 234. Check valve 242 prevents backflow from the mainline into the injection piping network while regulator 244 isolates thetracer injection system from fluctuations of pressure in the main gasline. Vent 246 is provided to vent that portion of the system downstreamof the check valve 242, while an isolation valve 248 is provided toisolate the entire gas injection system piping network from the pipeline256.

When the instant method is being used to tag methane in an undergroundstorage field, the tracer compound may be added continuously to all themethane as it is pumped into the storage field. Alternatively, only a"slug" of methane being pumped into the storage field may be labeled,leaving the tracer to diffuse from that slug throughout the storagefield. Obviously, when only part of the methane pumped into the storagefield is labeled, the concentration of tracer compound added to thatslug should be made sufficiently large that a regularly detectableconcentration of tracer is present after the tracer has become uniformlydiffused throughout all the methane in the storage field.

FIG. 1 shows schematically the apparatus used during testing forinjecting tracers into methane being stored in an underground storagefield. The apparatus was located adjacent the central compressor stationwhich supplied gas under pressure to the one or more injection wellsused for injecting gas into the storage field. A high-pressure gas tapto gas at about 1,000 psi. was connected on the upstream side of thetracer injection apparatus and pressurized methane therefrom passedthrough gas pressure regulators, which reduced the pressure of the gasin a pipe 10 to about 200 psi.

After passing a pressure regulator 16 and its accompanying pressureguage 20, the methane gas from the pipeline was then passed through asolenoid valve 22, which enabled the flow of gas from the pipeline to becut off when injection of gas into injection wells ceased, soterminating tracer injection. For this purpose, the solenoid valve 22was connected electrically to a differential pressure switch 24measuring the pressure differential (and thus flow rate) through anorifice 26 located adjacent an injection well. When the pressuredifferential across the orifice 26 fell below 12 inches water columnpressure, the solenoid valve 22 was closed, thus shutting down thetracer injection apparatus.

The stream of gas passes via the line 50 to a pressure regulator 52,which reduces the gas pressure in the line to about 50 psi. A pressuregauge 54 is provided downstream of the pressure regulator 52 in order toenable this pressure to be checked. Downstream of the pressure regulator52, the gas stream is divided by a T-piece 56 into two streams, one ofwhich passes via a valve 58 and a pump controller 60 to a pump 62, whichcontrols the flow of chloropentafluoroethane. The gas leaves the pump 62via a vent 64.

The other stream of gas from the T-piece 56 passes via a line 64,through a pump controller 66 to a pump 68 which controls the flow ofsulfur hexafluoride. The gas leaves the pump 68 via a vent 70.

In this apparatus, the tracer compounds are injected ino the methaneflow in the form of liquids, and accordingly, each of the pumps 62 and68 is a high-pressure piston-type positive displacement chemical feedpump of a standard commercial type which uses natural gas from the line10 as the pneumatic driving fluid. The pump controllers 60 and 66 allowthe throughput of the pumps 62 and 68 respectively to be varied fromfull flow to near zero by changing the piston stroke length and the pumpspeed.

The apparatus required for injection of chloropentafluoroethane andsulfur hexafluoride is simple, since both of these compounds can bemaintained in a liquid form in a closed container without refrigeration.The chloropentafluoroethane is stored in liquid form under a pressure ofabout 120 psi. in several large bottles 96, each equipped with amanually operable valve 98, which enables each bottle to be cut off fromthe remainder of the apparatus when it is necessary to remove anyparticular bottle for replacement. Each of the valves 98 opens into acollecting line 100, the pressure within which is monitored by apressure gauge 102. The line 100 feeds directly to the inlet side of thepump 62, which boosts the pressure of the liquid chloropentafluoroethaneto about 1,500 psi. and discharges it into a line 104 equipped with apressure guage 106, a relief valve 108 and a manually operable valve110.

The sulfur hexafluoride is stored in liquid form in a small bottle 112,which holds less than one pound (450 g.) of the tracer compound. Fromthe bottle 112, the sulfur hexafluoride passes via a manually operablevalve 114 and a line 116 to the inlet side of the pump 68, which boostsits pressure to about 1,500 psi. and feeds it into a line 118 equippedwith a pressure gauge 120, a relief valve 122 and a manually operablevalve 124. It will be appreciated that the manually operable valves 110and 124 allow each of the tracers to be switched on and off as desired,thereby enabling any desired combination of the two tracers to beinjected.

The lines 104 and 118 carrying the tracer compounds under pressure mergeat a T-piece 126, at which the tracers are mixed. From this T-piece 126,the supply of tracers under pressure passes through a check valve 128(which serves to prevent reverse flow of methane into the tracerapparatus should the pressure within the lines 104 and 118 beinsufficient), a back-pressure regulator 130, a relief valve 132 and amanually operable valve 134. The mixed tracers are finally discharged at136 into the pipeline supplying gas to the injection well, the tracersbeing fed into the pipeline at a point downstream of the point at whichthe driving methane gas for the apparatus is tapped. The pipeline intowhich the combined tracers are discharged at point 136 is a 24 inch (60centimeter) pipeline feeding all the injector wells within a particularfield. It will be appreciated that the test injection apparatus is fullyfunctional on any sized gas line.

Although sulfur hexafluoride is the most expensive tracer compound on aper unit weight basis, it is considerably cheaper in practice than theother two tracer compounds. For example, to tag two billion standardcubic feet of methane with sulfur hexafluoride at the preferredconcentration of 500 parts per trillion uses only 0.4 pound of thecompound, which at the commercial price of $12.40 per pound costs only$4.96. In contrast, to tag the same amount of gas usingchloropentafluoroethane at the preferred concentration of four parts permillion requires 3,333 pounds of the tracer compound. At the commercialprice of $2.04 a pound at which this compound is sold under the tradename Freon-115 by E. I. DuPont Company, this tracer costs $6,800.

EXAMPLE I

Using test injection apparatus similar to that described above, and asshown in FIG. 1, sulfur hexafluoride and chloropentafluoroethane tracerswere injected simultaneously into methane being introduced via a singleinjection well into an underground gas storage field. To compare theperformance of these tracer compounds with that of the prior art tracerethylene, ethylene was also added to the methane being pumped into thefield. Two other wells in the same field were shut off by blind flangesand checked daily for arrival of the tracers. One of these shut-in wellswas approximately 500 feet east of the injection well, while the otherwas approximately 500 feet north of the injection well. Over the courseof 10 days, approximately 42.5 million cubic feet of tagged methane wasinjected into the injection well. Although the concentration of thetracers varied somewhat from time to time due to experimentaldifficulties, the average tracer concentrations in the injected gas were36.4 parts per billion of sulfur hexafluoride, 3.5 parts per million ofchloropentafluoroethane and 73 parts per million of ethylene.

Before each daily test of the gas in each of the two recovery wells, gasin the tubing stream of each well was removed to obtain a fresh sampleof reservoir gas. A sample of the reservoir gas from each well was thenanalyzed by electron capture gas chromatography using an AnalyticalInstrument Development, Inc., Model 511-06 dual-column gas chromatographhaving a twelve foot chromosorb 102 column and a six-foot molecularsieve (5A) column which sufficed to differentiate between sulfurhexafluoride and chloropentafluoroethane. Ethylene which is notdetectable by electron capture gas chromatography was analyzed by usingthe same gas chromatograph as is above described in conjunction with a12 foot Chromosorb 102 column with a Hydrogen Flame Ionization Detector.

None of the tracer compounds was detected in either of the recoverywells during the first seven days of the experiment. On the 8th, 9th,and 10th days of the experiment, sulfur hexafluoride was recovered fromone recovery well at concentrations of 8, 13, and 10.5 parts per billionrespectively, and from the other recovery well at concentrations of 19,15, and 17.5 parts per billion respectively. The corresponding figuresfor days 8, 9, and 10 of the experiment for ethylene were 5, 5, and 7parts per million respectively in the first recovery well and 5, 5, and21 parts per million respectively in the second recovery well.Chloropentafluoroethane tracer was detected in both recovery wells ondays 8, 9, and 10 of the experiment; on day 8 the first recovery wellshowed only a tracer of this tracer, on day 9 the concentration ofchloropentafluoroethane were 0.16 parts per million, while nomeasurements were made on day 10 for the first recovery well. In thesecond recovery well, the concentrations of chloropentafluoroethanetracer were 0.5 and 0.75 parts per million on days 8 and 9 respectively;again, no measurements were made on day 10.

These results show that the two tracers used in the instant method movethrough the gas storage field at essentially the same speed as ethylene.In fact, the results appear to indicate that sulfur hexafluoride andchloropentafluoroethane more closely match the speed of movement ofmethane through the field than does ethylene, since once these twotracers have appeared their concentration is reasonably constant,whereas the ethylene, particularly in the second recovery well, wasobserved at relatively low concentrations before increasing to its finalconcentration, thus indicating that ethylene was "tied up" in thereservoir where sulfur hexafluoride and chloropentafluoroethane were notsimilarly tied up in the reservoir. The results also indicated that muchsmaller tagging concentrations of sulfur hexafluoride would havesufficed as tracer, since the amounts of sulfur hexafluoride measured atthe recovery wells were at least two orders of magnitude greater thanthe minimum concentration at which sulfur hexafluoride was detectable bythe apparatus used.

EXAMPLE II

In this experiment which again utilized the test injection equipmentschematically shown in FIG. 1, the gas storage field used had only asingle well which was used for both injection and withdrawal of the gas.In order to quantify the amount of tracer compounds which could berecovered from the reservoir, and the mixing of tagged and untagged gaswithin the reservoir, a slug of 308 thousand standard cubic feed of gascontaining (on average) 3 parts per trillion of sulfur hexafluoride and0.6 parts per million of chloropentafluoroethane was injected into thewell; thereafter 840 thousand cubic feet of untagged methane wasinjected into the field over a period of 16 hours, the injection ratethus being 1.75 million standard cubic feet per day. After injection hadfinished, the gas was allowed to remain in the field for twelve hoursand then gas was withdrawn and continuously monitored for the presenceof the two tracers in order to determine the total amount of gascontaining detectable concentrations of the two tracers and theproportion of the tracer injected which is recovered.

Sulfur hexafluoride was found to be the most mobile tracer, being foundin 329% more gas than originally tagged. Sulfur hexafluoride also showeda good recovery rate, 86% of the tracer injected being recovered.Chloropentafluoroethane was detected in 294% more gas than wasoriginally tagged, and at a recovery rate of 60%.

These results indicate that the tracers are somewhat less mobile thanethylene, since in similar earlier tests with ethylene, ethylene wasfound in 650% more gas than was originally tagged and showedsubstantially complete recovery.

In a second test, 411 thousand cubic feet of tagged gas containing 640parts per trillion of the sulfur hexafluoride and 3.3 parts per millionof chloropentafluoroethane was injected into the field during a periodof four hours, and gas was withdrawn after only two hours delay. Becauseof the conditions, relatively little diffusion of the tracers intountagged gas occurred; the sulfur hexafluoride was detected in 75% moregas than tagged, the chloropentafluoroethane in 69% more gas thantagged. Reasonably good recovery was achieved for both tracers, therecovery rate being 80% for sulfur hexafluoride and 61% forchloropentafluoroethane. Ethylene gives substantially complete recoveryunder similar conditions.

Finally, in a third test 228 thousand standard cubic feet of tagged gascontaining 90 parts per trillion of sulfur hexafluoride and 0.44 partsper million of chloropentafluoroethane were injected into the field overa period of four hours, then the field was left unchanged for 98 days.During this lengthy period of storage, all the candidates tracers showedconsiderable diffusion into untagged gas; upon withdrawal, the sulfurhexafluoride was found in 477% more gas than originally tagged while thechloropentafluoroethane was each found in 208% more gas than originallytagged. Sulfur hexafluoride and chloropentafluoroethane showed apparentrecovery rates of 193% and 170% respectively (presumably the surplus wasdue to these tracers remaining from the earlier tests).

The third test shows that sulfur hexafluoride andchloropentafluoroethane have good permanence within gas storage fields,and thus are eminently suitable for tagging gas which may have to bestored for months within such field. The tracers tested showedconsiderable movement into untagged storage area of the reservoir duringthis long term test.

As a result of the foregoing tests, the following conclusions werereached:

1. Sulfur hexafluoride shows consistently high recovery rates and doesnot appear to be absorbed or removed from the gas phase in anysignificant quantities even after lengthy storage.

2. Chloropentafluoroethane shows good permanance in the long term test,comparable to that obtained with sulfur hexafluoride.

3. Both tracers appeared to be somewhat less mobile than ethylene, butthe test results indicated that they would all move satisfactorilythrough a gas field with the methane with which they were injected.

All of the analysis of tracer concentrations was done using the sameanalytical apparatus described in Experiment I above. The samples weretaken in the field by bottle sample and were taken to the laboratory foranalysis utilizing the equipment described. However a mobile apparatusutilizing the same basic equipment became available during the course ofthe testing and was utilized for the last few tests. It is anticipatedthat a mobile test unit of simple construction comprising the generalpieces of apparatus described at the pertinent point in Example I abovewill be utilized in all future testing and it would be anticipated thatline analysis in actual operations work would be done by such mobileequipment.

EXAMPLE III

Additional testing was done to determine the ability of the tracer gasesto diffuse through and mix with other gas already beinng stored in anunderground storage structure. The injection occurred into a number ofwells in a large field. The injection wells were isolated from the otherwells in the field by placing blind plates in the inner connectingsurface piping to ensure that any tracer arrivals at any of thewithdrawal wells would be through the reservoir rather than via thesurface piping. Preventing contamination via surface pipinng is a veryimportant consideration since such trace amounts of the tracer gases arerequired to do the tagging operation in the first place. The simplepresence of these tracer compounds in the piping can more thancontaminate a withdrawal sample.

Initially, 2.8 billion cubic feet of methane tagged with the tracers wasinjected into the field. This was followed by an injection of 7.7billion cubic feet of completely untagged gas, giving a total of 10.5billion cubic feet being injected into the underground structure. Thewithdrawal wells were located considerable distances away from theinjection wells toward the periphery of the field. The withdrawals werenot begun for a period of about five months. During the time immediatelyprior to the withdrawal cycle there were definitive detections of thetracers in concentration near some of the withdrawal wells. The tracersulfur hexafluoride was detected in six wells while the tracerchloropentafluoroethane was detected at only four of the well sites,sulfur hexafluoride being detected at all of the wells wherechloropentafluoroethane was detected while the reverse was not the case.

The arrival of sulfur hexafluoride alone at some of the wells suggeststhe following explanations:

1. The sulfur hexafluoride has greater mobility than thechloropentafluoroethane;

2. The chloropentafluoroethane was diluted to concentrations below itsdetectability; or

3. Low permeability of the underground structures restricted themigration of chloropentafluoroethane preventing concentrations atdetectable levels from moving to the samping sites.

It is believed that while chloropentafluoroethane shows fair mobilitycharacteristics, the structure of the rock in the area of the wells atwhich chloropentafluoroethane was not detected may have prevented thepermeation of the chloropentafluoroethane to those points. This leads tothe conclusion that sulfur hexafluoride is a better tracer gas thanchloropentafluoroethane, while chloropentafluoroethane is still apractical tracer in many of the underground reservoir sites that are nowin existance. The initial detection of the tracer gases at withdrawalwell before withdrawal began is primarily due to conduction due topressure drops through the reservoir and by diffusion due toconcentration gradients existing within the reservoir. This indicatesthat in fact the tracers were more than capable of diffusing into theuntagged gases.

The test analyses of the samples was conducted by a mobile gaschromatograph system of the electron capture variety; however, back-upconfirmation samples were taken to the laboratores and analyzed on thesame equipment described in Example I above.

When the withdrawal period began after 1 billion cubic feet of gas hadbeen produced from the storage, analysis was conducted at all of thewells. Sulfur hexafluoride was detected at every well throughout thewithdrawal sampling period over a period of two months. Because of theremoteness of the field from the laboratory that tests of the wells fortracer concentrations were conducted only every two weeks. The fact thattracers were detected immediately upon analysis following the withdrawalof 1 billion cubic feet from storage suggests that the tracers were welldiffused and mixed into the 7.7 billion cubic feet of untagged gas.Additionaly testing continued while an additional 9.2 billion cubic feetof gas were removed from storage. The sulfur hexafluoride andchloropentafluoroethane remained available in detectable concentrationsthroughout this withdrawal. Considering that a total of 9.12 cubic feetof sulfur hexafluoride vapor (3.5 pounds, or 1.5 kilogram of liquid)were injected into the 10.5 billion cubic feet of gas by means of the2.8 billion cubic foot spiking sample, the total concentration in thegas would be 0.87 ppb. The data obtained showed average concentrationsof sulfur hexafluoride ranging from 0.7 to 1.3 ppb during the entirewithdrawal period. This again confirms that the diffusion of the sulfurhexafluoride into the greater body of gas was very complete. This sameanalysis was not able to be performed on the chloropentafluoroethanebecause the concentrations were more erratic probably due to the lowerpermeability to that tracer in the underground structure that was testedat this time. Areas of low permeability will have had very little tracerconducted or diffused through them thereby allowing areas of higherpermeability to have even higher concentrations. The overallconcentration averages then depend on the mix of withdrawal wells in themain line at the time of sampling. However, the chloropentafluoroethanewas clearly shown to have good migration characteristics within thereservoir.

There are certain conditions which render the use of the instant tracercompounds in underground gas storage fields more difficult. For example,since all the tracer compounds (as well as the prior art tracerethylene) have some solubility in petroleum distillates, where suchdistillates are present in a gas storage field the distillates willabsorb some of the tracer making the tracer more difficult to detect. Insome circumstances it may be desirable to increase the tracerconcentrations in order to allow for losses of tracer to distillates.Laboratory tests establish that sulfur hexafluoride has a solubility inpetroleum distillates about equal to that of the prior art tracerethylene, while chloropentafluoroethane is slightly more soluble thanethylene in petroleum distillates. In addition, when tagging gas beingpumped into large gas storage fields, care should be taken to avoidexcessive dilution of the tagged gas with untagged gas, to a point atwhich the tracers fall below detectable concentrations. For example, inExample I above, approximately 4.2 million standard cubic feet of taggedmethane were injected into a field of 1.2 billion standard cubic footcapacity; thus, the tagged gas amounted to 0.35% of the field'scapacity. As described above, the tests set forth in Example I producedhighly acceptable results. In contrast, in one unsuccessful experiment17 million standard cubic feet of tagged gas containing (on average) twoparts per billion of sulfur hexafluoride and 2.1 part per million ofchloropentafluoroethane were injected into a field having a capacity of23.7 billion standard cubic feet which contained some light oils andpetroleum distillates; thus the tagged portion of gas amounted to only0.07% of the capacity of the field. Following injection of this taggedgas an additional 25 million standard cubic feet of untagged gas waspumped into the field via the same well. Tests for the tracer compoundswere conducted at the gas issuing from a production well approximatelyone half mile from the injection well over a period of seven months butno trace of the compounds was detected. Also, tests at several otherwithdrawal wells in the same gas storage field proved negative afterseven months. Finally, when after seven months gas was withdrawn fromthe injection well, sulfur hexafluoride was detected in the gaswithdrawn at a level of 1.1 parts per billion, only 0.13 percent of theinjection level (allowing for dilution of the sulfur hexafluoride by theuntagged gas which was followed after the tagged gas, the injectionlevel of sulfur hexafluoride was 840 parts per billion). Duringcontinued withdrawal from the injection well, the concentration ofsulfur hexafluoride rose only to ten parts per billion, 1.2% of theinjection level.

The relative lack of success of this test may be due in some degree tothe absorption of the tracers into the distillates present in the gasfield, but is likely that the major factor accounting for the relativelack of success is excessive dilution of the tagged gas injected by themuch larger quantities of gas already present in the very large field.Absorption of the tracers into formations present in the field andadjacent areas may also play some part in the failure of this test.

It will be appreciated that numerous changes and modifications may bemade in the above-described embodiments of the invention withoutdeparting from the scope thereof. Accordingly, the foregoing descriptionis to be construed in an illustrative and not in a limitative sense, thescope of the invention being defined solely by the appended claims.

We claim:
 1. A method for tagging methane during underground storage ofmethane, said method comprising adding to said methane a tracercompound, said tracer compound having the properties of (1) notoccurring in natural supplies of methane, (2) diffusing through anyunderground methane storage field in a manner very similar to methane,and (3) substantially insoluble in petroleum distillates, said compoundbeing selected from the group consisting of sulfur hexafluoride andchloropentafluoroethane, and thereafter passing said methane in gaseousform into an underground gas storage field having petroleum distillatestherein,the amount of tracer compound passage into the field being greatenough that said tracer may be detected after said tracer is uniformlydisbursed throughout the field.
 2. A method according to claim 1 whereinsaid tracer compound is sulfur hexafluoride.
 3. A method according toclaim 2 wherein said sulfur hexafluoride is added to said methane in theform of a liquid solution of sulfur hexafluoride in a liquid solvent. 4.A method according to claim 3 wherein said solvent is methanol.
 5. Amethod according to claim 3 wherein said solvent is carbon dioxide.
 6. Amethod according to claim 2 wherein sulfur hexafluoride is added to saidmethane in the form of a gaseous mixture comprising sulfur hexafluorideand a carrier gas.
 7. A method according to claim 6 wherein said carriergas is nitrogen.
 8. A method according to claim 2 wherein said sulfurhexafluoride is added to said methane in an amount not exceeding about 5parts per billion by volume, when said methane and said sulfurhexafluoride are both in gaseous form.
 9. A method according to claim 1wherein said methane is in gaseous form.
 10. A method according to claim1 wherein said tracer compound is chloropentafluoroethane and is addedto said methane in an amount not exceeding about 20 parts per million byvolume, when said methane and said chloropentafluoroethane are both ingaseous form.
 11. A method according to claim 1, further comprisingrecovering a sample of gas from said underground gas storage field, andtesting said sample for the presence of said tracer compound.
 12. Amethod according to claim 11 wherein said testing for said tracercompound is effected by electron capture gas chromatography.